1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Discussion of the Background
As an image bearing member for use in an image forming apparatus applied to a photocopier and a laser printer, inorganic image bearing members formed of, for example, selenium, zinc oxide and cadmium sulfide, used to be widely used. However, in terms of the reduction of burden on environment, cost reduction and design latitudes, organic image bearing member (e.g., organic photoconductors (OPC)) have been diffusing now.
This organic image bearing member can be classified by layer structure. For example, organic image bearing members can be typified into (1) uniform single layer type, in which photoconductive resin (e.g., polyvinyl carbazol (PVK)) or charge transfer complex ((e.g., PVK-TNF (2,4,7-trinitrofluorenone)) is provided on an electroconductive substrate, (2) dispersion single layer type, in which a dispersion where a pigment (e.g., phthalocyanine and perylene) is dispersed in a resin is provided on an electroconductive substrate, and (3) layer accumulation type, in which a photosensitive layer provided on an electroconductive substrate is functionally separated into a charge generation layer (CGL) containing a charge generating material and a charge transport layer (CTL) containing a charge transport material.
The layer accumulation type is advantageous in terms of high sensitivity and has a high design latitude for high sensitivity and high durability. Thus, currently, most of the organic image bearing members adopts this layer structure.
The characteristics of these organic image bearing members are highly dependent on materials, especially charge transfer materials. The charge transfer materials are typified into positive hole transfer materials having a function of transferring positive holes, and electron transfer materials having a function of transporting electrons. Among these, electron transfer materials are demanded because the above-mentioned structure is positively charged, meaning generation of ozone and non-uniformity of charging are little in comparison with negative charging.
However, it is extremely difficult to find excellent electron transfer materials as materials for an image bearing member. For example, there are materials having excellent electron transfer properties with a problem about safety, for example, mutagenic property. Such materials are not suitable for practical use.
In addition, there are electron transfer materials which have no problem about safety but about stability after formation of a layer. To be specific, charging property and potential maintaining property in a dark place and quick light attenuation characteristics during irradiation are gradually lost. Such a material is dissolved over time so that the characteristics thereof deteriorate. This leads to degradation of image quality due to extreme deterioration of electric characteristics over repetitive use.
The issue is that the quality of images is initially good but deteriorates over a long period of time. Therefore, for example, unexamined published Japanese patent application No. (hereinafter referred to as JOP) describes a technology in which various kinds of additives are added in layers of an image bearing member.
However, such additives inhibit electron transfer property inherently seen in an electron transfer material. This causes a side effect, for example, deterioration of sensitivity characteristics. When the addition amount of such an additive increases, a binder resin becomes brittle. This reduction in strength leads to deterioration of anti-abrasion property. Consequently, there is no image forming apparatus which produces quality images for a long-time repetitive use.